Cleavage of fB by fD results in formation of the initial AP C3 convertase C3(H2O)Bb, which, like the classical C3 convertase C4bC2a, can cleave C3 into C3b and C3a. The generation of C3b allows the AP to be fully activated via formation of the bona fide AP C3

convertase Stem Cell Compound Library C3bBb (Fig. 1). Newly formed C3bBb is stabilized by the plasma protein properdin that binds to the complex and slows its deactivation.4 In fact, it should be noted that while the spontaneously generated C3(H2O)Bb is unique to AP, the C3b fragment generated from any of the pathways can bind to fB and, with the participation of fD, can form the AP C3 convertase C3bBb, which serves as an amplification loop for the entire complement system by rapidly augmenting the conversion of C3 to C3b necessary for full activation of the system and its downstream effects (Fig. 1).4 The cleavage of C3 to C3b is therefore the key step of convergence in the activation of the complement cascade.3

Apart from initiating the AP complement, C3b attaches to cells or immune complexes through covalent bonding; the opsonization of these targets by C3b or its further cleavage fragments facilitates their transportation and disposal through the endoreticular system. Additionally, C3b can associate with either of the C3 convertases to form the C5 convertase that cleaves C5 into C5a and C5b and initiates the terminal complement cascade, ultimately resulting in the formation of the multimeric membrane attack complex (MAC) (Fig. 1). In contrast to the early steps of complement activation,

assembly of the cytolytic MAC on the cell surface Selleckchem PLX4032 is a Baf-A1 chemical structure nonenzymatic process, initiated by association of C6 and C7 to C5b and subsequent insertion of the C5b-7 complex into the cell membrane through a hydrophobic domain in C7.5 Further attachment of C8 and multiple copies of C9 to the membrane-residing C5b-7 leads to assembly of the MAC, which creates physical pores in the cell membrane and causes lysis.3,5 Although the above scheme of complement activation is well established, two recent findings have provided novel insight into the activation mechanism of the AP. Biochemical and gene-targeting studies have revealed a critical role of properdin in initiating AP complement activation on some, although apparently not all, susceptible surfaces.6–10 Accumulating evidence supports the conclusion that, in addition to serving as a stabilizer of C3bBb, properdin can function as a pattern recognition molecule to trigger AP complement activation and in some instances such an activity of properdin is indispensible for the AP.6,7 The second notable finding of recent studies is the requirement of MASP1/3 for normal AP complement activity.11 It has been shown that MASP-1/3 cleaves inactive fD zymogen into the active form of fD that is normally present in plasma.